Add MessageType filter to FindInbox to exclude legacy VERSION_UPDATE
notifications from inbox queries. This resolves the issue where users
saw notification counts but no items displayed, as VERSION_UPDATE
entries cannot be rendered in the new UserNotification API.

Changes:
- Add MessageType field to FindInbox struct for database-level filtering
- Implement JSON extraction filters in SQLite, MySQL, and PostgreSQL drivers
- Update ListUserNotifications to filter MEMO_COMMENT type at store level

This approach improves performance by filtering at the database rather
than in application code, reducing unnecessary data transfer for users
with many legacy inbox entries.

Fixes #5278

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Co-Authored-By: Claude <noreply@anthropic.com>